In the production of Gray Iron Castings, the common casting defects are: stomata, composition and performance failure, hot cracking and cold cracking, shrinkage and shrinkage, slag and iron beans, cold and pouring insufficient, trachoma and sand, More meat and the wrong radiation, deformation and so on. Often, the causes of these defects are not only the problem of molding core, but also the problems of many production processes such as smelting pouring, sanding quality, sand removal and so on, and therefore must be analyzed in order to take appropriate reasonable measures to solve them.
First, sand core and sand rigidity
Sand casting, due to the static pressure of the molten iron or solidification caused by the expansion of the force, often resulting in wall movement and sand core collapse, which will make the casting of the internal shrinkage and surface shrinkage. Therefore, in order to stabilize the casting size, it is necessary to maximize the mold.
In order to save the modeling materials, making a wide range of hollow core core, it is lighter than the solid core, so the heat capacity is small, solidification speed is slow, which will lead to sand expansion or sand core collapse. In addition, the molten iron may penetrate the hollow portion through the crack on the core or the core, which also causes the casting to be defective.
In order to improve the rigidity of the hollow sand core, wet sand or sodium silicate sand can be filled; can also be made in half of the shell core, the internal set of ribs, after the core can be bonded hard core.
Second, the correct choice of pouring temperature
1. Phenomenon may be formed when the pouring temperature is too low
(1) Manganese sulfide stomata This stomata is located below the casting surface and more in the above, often exposed after processing, stomatal diameter of about 2 ~ 6mm. Sometimes the pores contain a small amount of slag, metallographic studies have shown that this defect is formed by MnS segregation and slag mixing, because the pouring temperature is low, while the iron liquid containing Mn and S high.
(2) liquid slag processing after casting under the skin will find a single hole, the diameter of the hole is generally 1 ~ 3mm. In some cases there are only 1 to 2 holes. Metallographic studies have shown that these pores appear with a small amount of liquid slag, but there is no segregation of S.
(3) sand core gas caused by the pores and porous air holes often due to poor sand core caused by exhaust. Because the core when the sand core in the core box in the hardening, which often makes the sand core exhaust number is not enough. In order to form the exhaust hole, the core can be added after drilling hardening.
The most common cause of pouring temperature is that before pouring, the molten iron is transported and stuck in the exposed ladle for long periods of time. With a ladle cover with a thermal insulation material, the heat loss can be significantly reduced.
Gray iron castings in the production of industrial machinery manufacturing widely used, there are many kinds of production processes, today, Xiaobian to take you to understand the Gray Iron Castings heat treatment process applications.
1, high temperature graphite annealing for the matrix structure contains more eutectic cementite castings to reduce the hardness and improve the machinability.
2, low temperature graphite annealing for gray iron castings hardness is too high, the matrix structure is not eutectic cementite, requires a high plasticity and high toughness of the gray iron castings.
3, to stress annealing for casting, casting welding, machining and other residual stress, to ensure that the gray iron castings and dimensional stability.
4, fully austenitized normalizing for the excessive amount of ferrite, the hardness of the Gray Iron Castings, to improve the strength of gray iron cast, hardness and wear resistance.
5, part of the austenitizing normalizing for the matrix is relatively uniform, and requires a certain strength and toughness of the gray iron castings.
6, complete austenitic quenching, tempering for the use of different tempering temperature of the matrix structure, to improve the gray iron castings strength, hardness and wear resistance.
7, complete austenite isothermal quenching for obtaining bainite matrix, improve the overall performance of gray iron castings.
8, surface hardening used to improve the strength of gray iron casting, surface strength and wear resistance.
9, chemical heat treatment for the gray iron castings surface special physical, chemical and mechanical properties.